Effect pigments, also known as nacreous pigments and pearlescent pigments, are lustrous specialty chemical products that are widely used in a variety of high-end applications such as automotive finishes. Effect pigments simulate pearly luster or have effects that range from silky sheen to metallic luster as disclosed in L. M. Greenstein, “Nacreous (Pearlescent) Pigments and Interference Pigments”, The Pigment Handbook, Volume 1, Properties & Economics, Second Edition, Edited by Peter A. Lewis, John Wiley & Sons, Inc. (1988). One of the attributes of the effect pigments is that they can generate a range of optical effects depending on the angle at which they are viewed.
Pearlescent or nacreous pigments simulate the effect of natural pearl and are composed of thin platelets which are transparent in the visible region of the spectrum. The platelets are very smooth and part of the light which strikes the platelets is reflected and part is transmitted through the platelets. That part of the light that is transmitted is subsequently reflected by other layers of platelets. The result is that multiple reflections from many layers occur and this results in depth of sheen since the eye cannot focus on one particular layer.
The reflection that occurs is specular in that the angle of incidence equals the angle of reflection. The amount of light reflected at non-specular angles is small and the amount of light reflected diminishes very quickly as the specular angle is passed. The result is that pearlescent pigments are extremely sensitive to viewing angle. In order for the maximum amount of light to be reflected, the platelets must be extremely smooth. Any surface roughness causes light to be scattered in a non-specular manner and diminishes the lustrous effect.
The platelets must be aligned parallel to each other and to the substrate for maximum reflectivity. If not so aligned, light will be reflected randomly and again, luster will diminish. The amount of light that is reflected depends on the index of refraction. As the index of refraction increases, the amount of reflected light increases.
However, in a number of applications, the effect materials have a lesser degree of hiding power than desired. To remedy this problem, a variety of materials have been incorporated in effect pigment formulations.
Effect pigments are often based on platelet shaped particles. Because the optical effect is the result of multiple reflections and transmission of light, it is desirable to provide particles that will align in the medium in which they are found and to optimize the desired effect. The presence of either misaligned particles or particles of an additive, or both, interferes with this objective and diminishes the optical effect of the pigment. It is therefore generally considered to be desirable for any additive being used for increased hiding to be somehow bound to the platelets rather than present as part of a physical mixture.
Effect pigments, particularly pigments based on mica, have long been used in automotive top coats in order to achieve a colored metallic effect, among other reasons. That metallic effect can be characterized by the flip-flop of light to dark as the viewing angle is changed. In the case of mica pigments, that flip-flop is from the reflection color of the mica to dark. Most automotive top coats are required to be opaque to ultraviolet light and also to visible light when applied at a conventional thickness of about 0.5 to 1.2 mils (about 12.7-30.5 μm) for a variety of reasons. It has been a challenge to maintain the “face” or reflection color which is contributed by the mica pigment while at the same time developing hiding in that it is known that opaque pigments greatly reduce the color/effect of the mica pigments.
Metallic flake pigments such as aluminum are opaque to light, i.e., no light is transmitted. Because of the preceding property, metallic flake pigments cover well and thus, the substrate over which they are coated can be completed hidden. This property is known as hiding power.
Blends of aluminum metal pigments with mica pigments (such as TiO2-coated mica pigment) are well known. For instance, U.S. Pat. No. 6,503,965 teaches an ink which can contain a non-fluorescent pigment alone or a mixture of two or more non-fluorescent pigments which can be selected from a long list of such pigments, including aluminum flake pigments (with thickness ranging from about 0.1 to about 2 microns) and TiO2- and Fe2O3-coated mica pigments. U.S. Pat. No. 2,278,970 teaches that thin mica flakes are suitable for use as an inert filler in combination with aluminum flake pigment to extend the covering quality of the latter. U.S. Pat. No. 6,331,326 teaches coating a primer and/or a first metallic paint containing a non-leafing type aluminum flake, and then applying a second metallic paint containing small thin flakes. The primer can be blended with a flat pigment, such as the thin aluminum flakes as well as flaky mica, to increase hiding power or hiding sand scratches on the substrate.                U.S. Pat. No. 6,306,931 teaches the use of preferred aluminum flake pigments that have median particle size of about 100 microns or less or especially 10 microns or less for incorporation into a coating. U.S. Pat. No. 6,398,861 teaches the use of an aluminum flake pigment having a diameter range of 6 microns to 600 microns for coatings.        Silberline.com advertises that its vacuum metallized aluminum flake can be used in cosmetics to achieve a smooth, mirror-like metallic effect and to deliver highly reflective, brilliant finishes.        
Blends of aluminum with non-effect materials are also known. For example, U.S. Pat. No. 4,937,274 teaches mixing aluminum flake pigments with ultrafine materials such as titanium dioxide. This coating composition does not include any interference (effect) mica pigments but is said to be still capable of providing an effect like those coatings containing the interference mica pigments and aluminum flake.
U.S. Pat. No. 6,267,810 teaches the use of a pigment comprising 15 grams of Cromal IV (Eckart) Al 14-18 microns and 15 grams of Fe2O3-coated Al2O3 flakes of particle size 5-60 microns in a printing ink in Example 2. No mention is made as to the effect, if any, provided by this pigment.
On May 11, 2004, Eckart (see eckartamerica.com) reported that METALURE® pigments are microscopically thin aluminum platelets manufactured in a physical vapor deposition process to provide pigments with uniform thickness and homogeneous smooth surface. The average particle size range was 10-13 microns. On May 11, 2004, it was reported that Eckart's METALURE® ultra brilliant aluminum dispersion when mixed with pearlescent provides polychromatic effects. The article is silent on the thickness of the aluminum pigment or the percentage of aluminum pigment used in combination with pearlescent (see cosmetics.com).
The provision of new high hiding power compositions is still desired and it is the object of the present invention is to provide such compositions. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.